drm/i915: remove "inline" keyword from ironlake_disable_display_irq

 - It's a static function
 - I just added a few more users to it
 - Its sister ironlake_enable_display_irq is not marked as inline
 - The compiler will still inline if it thinks it should do

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>